1. Field of the Invention
The present invention relates to a combustion control device and method for an engine, and more particularly to a combustion control device and method for an engine, which is capable of realizing a premix combustion that can sufficiently decrease the quantity of NOx (Nitrogen Oxides) and smoke in a wider operation region.
2. Description of the Related Art
In the conventional diesel engines, the combustion has been typically conducted by injecting fuel in the vicinity of the compression top dead center (TDC) of a piston when the temperature and pressure inside the cylinder (inside the combustion chamber) were high.
The injected fuel is mixed with the intake air, forming a mixture, this mixture is ignited, flame is formed, and the combustion is maintained by supplying subsequent injected fuel into the flame. In this combustion system, the ignition starts during fuel injection; in the present specification, this combustion system will be referred to as a normal combustion.
However, in recent years, new combustion systems have been suggested, those systems being capable of significantly reducing the quantity of NOx and smoke, without greatly degrading the fuel consumption, by setting the fuel injection timing earlier than the compression top dead center, thereby extending the ignition delay period and sufficiently enhancing the mixing of fuel and intake air (for example, Japanese Patent Applications Laid-open No. H9-112325 and 10-331690).
In those combustion systems, fuel injection is conducted within the interval from the intake stroke before the compression top dead center to the compression stroke and the ignition is started once a prescribed ignition delay period elapses after the fuel injection end. In such an injection system, the ignition delay period is long and the mixture is sufficiently leaned (rarefied) and homogenized. Therefore, local combustion temperature drops and the quantity of emitted NOx is reduced. Furthermore, smoke is also controlled because the combustion in a local air-deficient state is avoided. The combustion system in which the ignition is thus started after the fuel injection completion will be in the present specification referred to as a premix combustion.
Thus, though the premix combustion is effective in terms of improving the exhaust gas, because it can be employed only in a low-load and low-rpm range of the engine, it is presently necessary to switch to the normal combustion system in a high-load and high-rpm range. For example, the above-mentioned Japanese Patent Application Laid-open 10-331690 describes a diesel engine in which the premix combustion based on early injection is conducted during low-and medium-load operation, and the normal combustion in which fuel is injected in the vicinity of the compression top dead center is conducted during a high-load operation.
The main reason why the premix combustion cannot be employed in a high-load and high-rpm range is that the planned proper ignition cannot be ensured.
Thus, when the engine revolution speed is low, the fuel is at a high temperature and a high pressure inside the cylinder for a comparatively long time. As a result, chemical reactions easily proceed ensuring the ignition. By contrast, when the engine revolution speed is high, the fuel is at a high temperature and a high pressure inside the cylinder for a short time. As a result, a misfire easily occurs. In the case of premix combustion, because leaning and homogenization of the mixture smoothes out the temperature non-uniformity inside the cylinder, the transition to the expansion stroke can be made while a sufficient exothermic reaction has not yet been initiated in the beginning of combustion and a misfire can occur. For this reason, the misfire has been prevented by switching to the normal combustion, in which a spread occurs in the temperature distribution of the premix, in a high-rpm range.
Further, switching to the normal combustion was also made because conducting fuel injection early in the high-load range of an engine can cause severe diesel knocking.
An engine has recently been invented in which a pilot injection of a comparatively small quantity of fuel was conducted for premixing earlier than the compression top dead center and a main injection of a comparatively large quantity of fuel was conducted for the normal combustion in the vicinity of the compression top dead center (for example, Japanese Patent Applications Laid-open No 2000-145507 and 2001-207890).
With such a two-stage injection system, because the ignition is ensured by the main injection, the application in a wider operation range is possible.
However, because the fuel injected by the main injection is conducted by the normal combustion method, the quantity of emitted NOx and smoke is accordingly increased by comparison with the premix combustion based on single-stage injection.
Furthermore, the EGR (Exhaust Gas Recirculation (Recirculator)) by which exhaust gases are returned into the combustion chamber is effective for reducing the quantity of NOx, but if the EGR ratio in the normal combustion is increased, then air deficiency occurs and smoke is generated. Therefore, in the conventional two-stage injection system, it was necessary to control the EGR ratio. As a result, a certain quantity of NOx was also emitted. In this case, smoke and NOx are purified in after-treatment devices, but such a procedure increases fuel consumption and cost.
Further, even in the case of premix combustion using the above-mentioned single-stage ignition, if the EGR ratio was increased, the concentration of oxygen dropped and a misfire could even more easily occur. For this reason, a sufficient effect of reducing the emission of NOx by the EGR could not be obtained.